The present invention relates generally to electroactive polymers that convert from electrical energy to mechanical energy. More particularly, the present invention relates to pre-strained polymers and their use in actuators and various applications. The present invention also relates to compliant electrodes used to electrically communicate with electroactive polymers and methods of fabricating pre-strained polymers.
In many applications, it is desirable to convert from electrical energy to mechanical energy. Exemplary applications requiring translation from electrical to mechanical energy include robotics, pumps, speakers, general automation, disk drives and prosthetic devices. These applications include one or more actuators that convert electrical energy into mechanical workxe2x80x94on a macroscopic or microscopic level. Common electric actuator technologies, such as electromagnetic motors and solenoids, are not suitable for many of these applications, e.g., when the required device size is small (e.g., micro or mesoscale machines). These technologies are also not ideal when a large number of devices must be integrated into a single structure or under various performance conditions such as when high power density output is required at relatively low frequencies.
Several xe2x80x98smart materialsxe2x80x99 have been used to convert between electrical and mechanical energy with limited success. These smart materials include piezoelectric ceramics, shape memory alloys and magnetostrictive materials. However, each smart material has a number of limitations that prevent its broad usage. Certain piezoelectric ceramics, such as lead zirconium titanate (PZT), have been used to convert electrical to mechanical energy. While having suitable efficiency for a few applications, these piezoelectric ceramics are typically limited to a strain below about 1.6 percent and are often not suitable for applications requiring greater strains than this. In addition, the high density of these materials often eliminates them from applications requiring low weight. Irradiated polyvinylidene difluoride (PVDF) is an electroactive polymer reported to have a strain of up to 4 percent when converting from electrical to mechanical energy. Similar to the piezoelectric ceramics, the PVDF is often not suitable for applications requiring strains greater than 4 percent. Shape memory alloys, such as nitinol, are capable of large strains and force outputs. These shape memory alloys have been limited from broad use by unacceptable energy efficiency, poor response time and prohibitive cost.
In addition to the performance limitations of piezoelectric ceramics and irradiated PVDF, their fabrication often presents a barrier to acceptability. Single crystal piezoelectric ceramics must be grown at high temperatures coupled with a very slow cooling down process. Irradiated PVDF must be exposed to an electron beam for processing. Both these processes are expensive and complex and may limit acceptability of these materials.
In view of the foregoing, alternative devices that convert from electrical to mechanical energy would be desirable.
In one aspect, the present invention relates to polymers that are pre-strained to improve conversion between electrical and mechanical energy. When a voltage is applied to electrodes contacting a pre-strained polymer, the polymer deflects. This deflection may be used to do mechanical work. The pre-strain improves the mechanical response of an electroactive polymer relative to a non-strained polymer. The pre-strain may vary in different directions of a polymer to vary response of the polymer to the applied voltage.
In another aspect, the present invention relates to actuators comprising an electroactive polymer and mechanical coupling to convert deflection of the polymer into mechanical output. Several actuators include mechanical coupling that improves the performance of an electroactive polymer.
In yet another aspect, the present invention relates to compliant electrodes that conform to the changing shape of a polymer. Many of the electrodes are capable of maintaining electrical communication at the high deflections encountered with pre-strained polymers of the present invention. In some embodiments, electrode compliance may vary with direction.
In another aspect, the present invention provides methods for fabricating electromechanical devices including one or more electroactive polymers. Pre-strain may be achieved by a number of techniques such as mechanically stretching a polymer and fixing the polymer to one or more solid members while it is stretched. Polymers of the present invention may be made by casting, dipping, spin coating, spraying or other known processes for fabrication of thin polymer layers. In one embodiment, a pre-strained polymer comprises a commercially available polymer that is pre-strained during fabrication.
In another aspect, the present invention relates to a transducer for translating from electrical energy to mechanical energy. The transducer includes at least two electrodes and a polymer arranged in a manner which causes a portion of the polymer to deflect in response to a change in electric field. The polymer is elastically pre-strained.
In another aspect, the present invention relates to a transducer for converting from electrical energy to mechanical energy. The transducer comprises at least two electrodes and a polymer arranged in a manner which causes a portion of the polymer to deflect in response to a change in electric field provided by the at least two electrodes. The portion of the polymer deflects with a maximum linear strain between about 50 percent and about 215 percent in response to the change in electric field.
In yet another aspect, the present invention relates to an actuator for converting electrical energy into displacement in a first direction. The actuator comprises at least one transducer. Each transducer comprises at least two electrodes and a polymer arranged in a manner which causes a portion of the polymer to deflect in response to a change in electric field. The actuator also comprises a flexible frame coupled to the at least one transducer, the frame providing mechanical assistance to improve displacement in the first direction.
In another aspect, the present invention relates to an actuator for converting electrical energy into mechanical energy. The actuator comprises a flexible member having fixed end and a free end, the flexible member comprising at least two electrodes and a pre-strained polymer arranged in a manner which causes a portion of the polymer to deflect in response to a change in electric field provided by the at least two electrodes.
In another aspect, the present invention relates to an actuator for converting electrical energy into displacement in a first direction. The actuator comprises at least one transducer. Each transducer comprises at least two electrodes and a polymer arranged in a manner which causes a portion of the polymer to deflect in response to a change in electric field. The actuator also comprises at least one stiff member coupled to the at least one transducer, the at least one stiff member substantially preventing displacement in a second direction.
In yet another aspect, the present invention relates to a diaphragm actuator for converting electrical energy into mechanical energy. The actuator comprises at least one transducer. Each transducer comprises at least two electrodes and a pre-strained polymer arranged in a manner which causes a first portion of the polymer to deflect in response to a change in electric field. The actuator also comprises a frame attached to a second portion of the polymer, the frame including at least one circular hole, wherein the first portion deflects out of the plane of the at least one circular hole in response to the change in electric field.
In another aspect, the present invention relates to an actuator for converting electrical energy into mechanical energy, the actuator comprising a body having at least one degree of freedom between a first body portion and a second body portion, the body including at least one transducer attached to the first portion and the second portion, each transducer comprising at least two electrodes and a pre-strained polymer arranged in a manner which causes a portion of the polymer to deflect in response to a change in electric field; the actuator also comprising a first clamp attached to the first body portion and a second clamp attached to the second body portion.
In yet another aspect, the present invention relates to an actuator for converting electrical energy to mechanical energy, the actuator comprising a transducer, the transducer comprising a polymer arranged in a manner which causes a first portion of the polymer to deflect in response to a change in electric field, a first electrode pair configured to actuate a second portion of the polymer and a second electrode pair configured to actuate a third portion of the polymer, the actuator also comprising an output member coupled to a first portion of the polymer.
In another aspect, the present invention relates to an electrode for use with an electroactive polymer. The electrode comprises a compliant portion in contact with the electroactive polymer, wherein the compliant portion is capable of deflection with a strain of at least about 50 percent.
In yet another aspect, the present invention relates to an electrode for use with an electroactive polymer. The electrode comprising a compliant portion in contact with the electroactive polymer, wherein the electrode comprises an opacity which varies with deflection.
In yet another aspect, the present invention relates to an electrode for use with an electroactive polymer. The electrode comprising a compliant portion in contact with the electroactive polymer, wherein the compliant portion comprises a textured surface.
In another aspect, the present invention relates to a method of fabricating a transducer including a pre-strained polymer. The method comprises pre-straining an electroactive polymer to form the pre-strained polymer. The method also comprises fixing a portion of the pre-strained polymer to a solid member. The method additionally comprises forming one or more electrodes on the pre-strained polymer.
In still another aspect, the present invention relates to a method of fabricating a transducer comprising multiple pre-strained polymers. The method comprises pre-straining a first polymer to form a first pre-strained polymer. The method also comprises forming one or more electrodes on the first pre-strained polymer. The method further comprises pre-straining a second polymer to form a second pre-strained polymer. The method additionally comprises forming one or more electrodes on the second pre-strained polymer. The method further comprises coupling the first pre-strained polymer to the second pre-strained polymer.
These and other features and advantages of the present invention will be described in the following description of the invention and associated figures.